The proportional (P), integrating (I) and derivative (D) type control is still widely utilized in the present industrial process control due to its simplicity and robustness, even though a variety of theories and control strategies have been developed in the recent years. Moreover, a conventional PID type controller is still being used even if the controller is based on a microcomputer, since persons in the industry skilled in the art have a long and experienced knowledge about the tuning of such PID controllers.
Theoretically, the parameters of a controller can be set at the same time of designing he control system; however, most of the controller parameters are tuned in accordance with the characteristics of the process after the controller is installed. There are two tuning methods which are adapted much more frequently than the other tuning methods in the art, which are
a) the method proposed by Ziegler and nichols in 1942, which comprises gradually increasing the P controller gain as the I and D control units are disconnected in a closed loop until the controlled variable undergoes a sustained oscillation, and then calculating the parameters of the PID controller; and
b) the step test method which comprises placing the controller in the manual mode and making a small step change in the controller output, approximating the resulting process reaction curve by a simple dynamic model, and then determining the parameters of the controller.
The Ziegler and Nichols method suffers at least the following two disadvantages:
i) It forces the process into a condition of marginal stability which may lead to unstable operation, due to process changes or external disturbances.
ii) The loop-tuning method is quite time-consuming since a trial and error procedure is employed to obtain a sustained oscillation.
The chief disadvantage of the b) method is that the experimental test is performed during open-loop operation and the tuning results will be very sensitive to unanticipated disturbances.
Hagglund and Astrom, in U.S. Pat. No. 4,549,123, disclose an automation method for tuning a PID-type controller, wherein a relay is connected in series to the process in a closed loop P control such that he process is forced into a condition of self-oscillation. This method is much more easier and simple than that of Ziegler and Nichols; however, the fluctuating change between open and close of the relay is not suitable for certain types of process controls, such as fuel flow rate control in a combustion process, because of the safety consideration.
The purpose of the present invention is to provide an improved tuning method for PID controller which does not have the drawbacks described above.